The present invention relates to a multiple filter unit for purifying air, in particular for motor vehicles or closed environments, and insertable into ventilation and air conditioning systems. Technical development has inevitably led to chemical, physical or biological alterations of the environment. Atmospheric pollution is itself a chemical-physical alteration of the air. The alteration is natural when it relates to bacteria, gas of volcanic origin, marine aerosols and the like. In contrast, it is artificial if due to combustion systems, industrial plants or motorisation. Any type of combustion leads to final products which have to be dispersed and settled. These are constituted by oxygenated carbon compounds, unburnt hydrocarbons, nitrogen oxides, oxygenated sulphur compounds, water vapour and unburnt or incombustible solid particles. With regard to industrial plants, the fact that they are located in relatively small determined areas leads to a very high concentration of industrial emissions. The main industrial pollutants are oxygenated carbon compounds, nitrogen oxides, sulphur trioxide, hydrocarbons and solid particles. Motor vehicle engines themselves emit large quantities of pollutants, especially in the gaseous state, including carbon monoxide, unburnt hydrocarbons and nitrogen oxides. There are various pollutant removal methods, which can be grouped into five categories: (a) conversion of the gaseous pollutant into another product and recovery of the new substance; (b) decomposition of the gaseous pollutants and conversion into innocuous products; (c) conversion into easily eliminated products; (d) elimination of gaseous pollutants by cooling and condensation; (e) elimination by absorption in liquid or adsorption on a solid.
Studies carried out in Italian urban areas have demonstrated the influence which atmospheric conditions and seasonal cycles have on pollution. In this respect, particularly in winter, when wind is absent a stable system forms which tends to trap emissions, which are essentially of domestic (heating) and vehicle origin. As the day progresses, pollution high points are observed depending on the particular moment. In contrast, during summer all values decrease and are more uniform. Again, pollution is not uniform over the entire urban area, but depends on the closeness or otherwise of industries, the roads of greatest circulation, and whether tree-containing areas are present. It is however general, and extends also to roads outside the town, where it is naturally proportional to the intensity of the traffic using them.
In order to combat pollution, industries already use purifiers. Special foamed material is able to retain particulate matter. Filters with activated carbon are used on cooker hoods without a discharge duct. The army, the fire service and police use protective masks for high gas, fume and dust concentrations. Catalysts for application to the exhaust pipes of internal combustion engines using leadless petrol have been designed in order to prevent their very rapid deterioration. In large, closed environments such as factories, cinemas, restaurants and the like, central air treatment plants are already installed, but are generally limited to solid particulate matter, so that the air which is introduced is not clean but contains pollutant substances. The air normally breathed in automobiles, offices and small closed environments is also polluted by the air originating from the outside.